Problem: Nauuo is a girl who loves playing chess.

One day she invented a game by herself which needs n chess pieces to play on a m× m chessboard. The rows and columns are numbered from 1 to m. We denote a cell on the intersection of the r-th row and c-th column as (r,c).

The game's goal is to place n chess pieces numbered from 1 to n on the chessboard, the i-th piece lies on (r_i,\,c_i), while the following rule is satisfied: for all pairs of pieces i and j, |r_i-r_j|+|c_i-c_j|≥|i-j|. Here |x| means the absolute value of x.

However, Nauuo discovered that sometimes she couldn't find a solution because the chessboard was too small.

She wants to find the smallest chessboard on which she can put n pieces according to the rules.

She also wonders how to place the pieces on such a chessboard. Can you help her?

Input

The only line contains a single integer n (1≤ n≤ 1000) — the number of chess pieces for the game.

Output

The first line contains a single integer — the minimum value of m, where m is the length of sides of the suitable chessboard.

The i-th of the next n lines contains two integers r_i and c_i (1≤ r_i,c_i≤ m) — the coordinates of the i-th chess piece.

If there are multiple answers, print any.

Examples

Input


2


Output


2
1 1
1 2

Input


4


Output


3
1 1
1 3
3 1
3 3

Note

In the first example, you can't place the two pieces on a 1×1 chessboard without breaking the rule. But you can place two pieces on a 2×2 chessboard like this:

<image>

In the second example, you can't place four pieces on a 2×2 chessboard without breaking the rule. For example, if you place the pieces like this:

<image>

then |r_1-r_3|+|c_1-c_3|=|1-2|+|1-1|=1, |1-3|=2, 1<2; and |r_1-r_4|+|c_1-c_4|=|1-2|+|1-2|=2, |1-4|=3, 2<3. It doesn't satisfy the rule.

However, on a 3×3 chessboard, you can place four pieces like this:

<image>

Solution (in PYTHON3):

Solution:
def get_int_list():
    return list(map(int, input().split()))

def get_max_repeats(l):
    if len(l) == 0:
        return 0
    repeats = 1
    max_repeats = 1
    last_element = l[0]
    for i in range(1, len(l)):
        if l[i] == last_element:
            repeats += 1
        else:
            if repeats > max_repeats:
                max_repeats = repeats
            repeats = 1
        last_element = l[i]
    if repeats > max_repeats:
        max_repeats = repeats
    return max_repeats
from math import ceil

n = int(input())

m = ceil((n - 1) / 2) + 1
print(m)
places = []
count = 0
for i in range(m):
    if count < n:
        places.append((1, i+1,))
        count += 1
for i in range(1, m):
    if count < n:
        places.append((i+1, m,))
        count += 1

for place in places:
    print(str(place[0]) + " " + str(place[1]))